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1. Journal of Environment and Earth Science
ISSN 2224-3216 (Paper) ISSN 2225-0948 (Online)
Vol. 3, No.12, 2013
www.iiste.org
Determination of Mercury Level in Rana esculenta (Frog),
Sediment and Water from River Guma, Benue State Nigeria
Ugbidye Shaapera, Ishaq S. Eneji* and Rufus Sha’Ato
Department of Chemistry University of Agriculture Makurdi, Benue State Nigeria.
*corresponding author: eneji3@yahoo.com
Abstract
The level of mercury was determined in R. esculenta (edible frog), sediment and water from river Guma, Benue
State, for three (3) consecutive months using hydride generation atomic absorption spectrophotometer (HG-AAS)
technique. The mean concentrations of mercury in the R. esculenta, water and sediment were 0.027mg/kg,
0.00mg/kg and 0.001mg/kg, respectively. The absence of mercury in the water signifies its affinity to adsorbed
to any surface in the river. Mercury builds up in the tissues of R. esculenta and its levels in tissues increase as we
go up the food chain. The result of the analysis shows that the level of mercury is always higher in the liver
(0.014mg/kg) compared to intestine (0.010mg/kg) and muscle (0.003mg/kg). The mean concentration of
mercury obtained in R. esculenta (0.027mg/kg) was below the International Atomic Energy Agency
recommendation value (IAEA – 433) of 0.168mg/kg.
Keywords: Mercury, Edible Frog, Sediment, Guma, HG-AAS
1.0
Introduction
Today, R. esculenta also known as edible frog has become the main supply of protein besides meat and poultry.
Countries like France, Japan, China, Thailand, Indonesia and Nigerian, take edible frog as the main dish of their
diet because it provides protein (Dural et al., 2007). Therefore, their mode of feeding and environment to these
frogs needs investigation. Heavy metals have the tendency to accumulate in the various aquatic animals and the
accumulation depends on the intake and elimination from the body (Karadede et al., 2004). Marine fish and
edible frog were exposed to these metals that human being consumed as sea foods. Therefore, there is a link for
the transfer of toxic metals into human beings as we go up the food chain. However, frog also may contain
chromium, mercury and lead that could give negative effects for health. The marine organisms accumulate
contaminants such as metals from the environment and have been extensively used in marine pollution
monitoring (Mora et al., 2004). These metals accumulate in frog from water, food, sediment and some suspended
particulate matter (Agusa et al., 2005). The contamination of water bodies with heavy metals has become a
matter of concern over the last two decades (Voegborlo et al., 1999). The natural aquatic systems may
extensively be contaminated with heavy metals released from domestic, industries and other man-made activities
(Velez et al., 1998). Heavy metals contamination may have devastating effects on the ecological balance of the
environment and a diversity of aquatic organisms (Ashraj, 2005). Heavy metals are of particular concern due to
their potential toxic effect and ability to bioaccumulate in aquatic ecosystems (Censi et al., 2006). Heavy metal
concentrations in aquatic ecosystems are usually monitored by measuring their concentrations in water (Camusso
et al., 1995). Water quality standards should be applied to sediment because of its strong influence on the water
quality. However, total metal concentration in sediment is not a good estimation of bioavailability. Different
phases of sediment can vary in toxicity with the same concentration (Calmano et al., 1996). Even though,
mercury is a naturally occurring metal which has several forms of existence, the most common organic mercury
compound is methyl mercury, which is produced mainly by small organisms called bacteria in water and soil.
Methyl mercury builds up in the aquatic organisms and its levels in tissues increase as we go up the food chain.
Edible frog and other aquatic organisms’ intake are the major source of exposure to mercury, mainly in the form
of methyl mercury, which accumulates from surrounding waters (Rogers et al., 1992). Studies shows that edible
frog accumulate these heavy metals from the surrounding water bodies thereby leaving a health risk if taking as
food (US. DPHHS, 2005). EPA drinking water limit is 2ppb and FDA maximum permissible level of methyl
mercury in seafood is 1ppm. Therefore, methyl mercury is worse for young children than for adults, because
more of it passes into children’s brains where it interferes with normal development. In this study, we
investigate the distribution of mercury in edible frog, sediment and water from river Guma, Benue state, Nigeria.
The observed levels of this metal concentration were compared to the Provisional Tolerable Intake for mercury
as set by World Health Organization standard.
2.0
Materials and Methods
2.1
Sampling: Three sampling stations coded A (07.80691o North, 008.65763o East), B (07.80652o North,
008.65756o East) and C (07.76973o North, 008.59413o East) was established based on the anthropogenic
activities that are going on around the area. Samples of edible frog, water and sediments were collected from the
201

2. Journal of Environment and Earth Science
ISSN 2224-3216 (Paper) ISSN 2225-0948 (Online)
Vol. 3, No.12, 2013
www.iiste.org
River Guma for three consecutive months (between January and March, 2013). A total of 20 mature edible frog
samples with mean weight of 150 ± 3g and mean length 26 ± 2cm were obtained from the sampling station
(Figure 1). The samples were stored in an ice box in order to maintain the freshness and later transported (11/2
hours) to the laboratory for dissection to obtain muscles, intestine and liver. The edible frog samples organ
(muscles, intestine and liver) were oven dried separately for an hour to constant weight at 1050C. The organs
were pooled separately according to tissue type and milled with a mortar and pestle. They were put in dry
labeled plastic containers and stored in desiccators until digestion.
Figure 1: A typical picture of Edible frog showing the side view
Similarly, the sediment samples were taken with hand and transferred into polythene bag and transported to the
laboratory. The sediment was placed on a Formica surface ply wood board on a dust-free working bench and
spread to air-dry. The sediment was redistributed twice daily for effective drying. When dried, the sediment was
crushed in a mortar and sieved through the 2mm sieve into plastic containers and stored for subsequent analysis.
A procedure similar to that described by Poldoski (1980) was used to digest the samples. This involves digesting
10g portion of the ground samples with 10mL HNO3 and 2mL HClO4. The residue was dissolved and diluted
with 0.2% v/v HNO3 to 20mL and made up to 100mL with distilled water. The digest was stored in pre-cleaned
polyethylene bottles until analysis using hydride generation atomic absorption spectrophotometer. The KBH4,
carrier liquor and blank sample were connected into their respective sucking tubes. At the start of the hydride
generation (connected to the main AAS), the solutions were automatically suck into the system where the
mercury hydride was produced and transmitted to the electric quartz absorption tube and was detected and
recorded.
3.0
Results and Discussion
The results of analysis shows that the concentration of mercury in all the water samples from the three sampling
stations and between the periods of investigation (January, February and March) were below the detection limit.
Therefore, any Hg level found in the frog could be as a result of bioaccumulation.
The mean concentrations of mercury measured in the edible frogs are shown in figures 2 – 4. The concentration
of mercury varied in the organs from 0.001 – 0.006mg/kg in all the periods of investigation. Figure 2 illustrates
the mean concentration of mercury in liver, intestine and muscles of edible frogs from river Guma as obtained in
January, 2013. Similarly, figures 3 and 4 shows the mean concentrations of mercury for February and March,
respectively. However, in figure 4 the concentration of Hg in muscle of frog could not be detected, perhaps it
was below the detection limit. Even though, the concentrations of Hg obtained were generally about a factor ten
(10) higher than the recommended value of WHO within the period of this investigation.
202

3. Journal of Environment and Earth Science
ISSN 2224-3216 (Paper) ISSN 2225-0948 (Online)
Vol. 3, No.12, 2013
www.iiste.org
0.0045
Concentration (mg/kg)
0.004
Hg
0.0035
0.003
0.0025
0.002
0.0015
0.001
0.0005
0
liver
intestine
muscle
WHO
Figure 2: Mean January concentration of mercury in edible Frog organs from river Guma.
Concentration (mg/kg)
0.007
0.006
Hg
0.005
0.004
0.003
0.002
0.001
0
liver
intestine
muscle
WHO
Figure 3: Mean February concentration of Mercury in edible Frog organs from river Guma.
203

4. Journal of Environment and Earth Science
ISSN 2224-3216 (Paper) ISSN 2225-0948 (Online)
Vol. 3, No.12, 2013
www.iiste.org
0.0045
Concentration (mg/kg)
0.004
Hg
0.0035
0.003
0.0025
0.002
0.0015
0.001
0.0005
0
liver
intestine
muscle
WHO
Figure 4: Mean March concentration of Mercury in edible Frog organs from river Guma.
Generally, the result of the HG – AAS analysis shows that the level of Hg is always higher in the liver compared
to intestine and muscles. Mercury is rapidly absorbed and distributed by the blood; about 1% is deposited in the
brain where it is retained for a long time, and the rest is transported to the liver and kidneys where it is excreted
through bile and urine. The mean concentrations of Hg reported in this work are within the range of literature
values reported by previous studies. Mukherjee et al (2011) reported mercury concentration in Harpadon
nehereus, Daysciaen aalbida, pumpus argentius, Formio niger, Hilsa ilisha and Rastrellige kanagurta to be 0.91,
0.46 0.70 0.28, 0.37 and 0.93µg/g respectively in fishes from Bag Bergal, India. Voegborlo et al (2007) reported
mercury concentrations in fish species samples from the coastal waters of Ghana as in Lagolephalus
lagocephalus, Stromatteus fiatrla, Braelydenterus curitus, Pamulinus argus, Calappa rubroguhata, Gerres nigri,
Decapterus rhonchus, Braehydentera aurita, Diplodus puntazzo, parapristipomma humile, selene dorsalis,
Galeoides decadactylus, and Pseudotolithus senegalensis as 0.066, 0.004, 0.037,0.035, 0.056, 0.043, 0.070,
0.112, 0.034, 0.041 and 0.031µg/g. High value of mercury concentration of 0.32ppm in Lates nilotcus has been
reported on Kaduna river (Nwaedozie, 1998). Alinnor et al (2010) working on Nworier river, reported Hg level
of mean in Liza grandisaquamis and Sphyraena sphyraena to be 0.0083ppm and 0.0083ppm, respectively. Ekpo
et al (2008) reported mercury concentration in Metacembelus iconnbergii, Clarias lazera, Citarinus cithanus,
Tilapia Zilli and Erpetoicithy from Ikpo river in Benin City to be 0.004mg/kg, 0.003mg/kg, 0.003mg/kg,
0.00mg/kg and 0.002mg/kg respectively. Eneji et al (2011) also determined the concentration of these metals in
the gills, intestine and muscle tissues of two fish species; Tilapia Zilli and Clarias gariepinus obtained from up
and down streams of the River Benue. They reported the percentage composition of total heavy metals in the fish
organs to be 52.2% in the gills, 26.3% in the intestine and 21.5% in the muscle tissue in Tilapia Zilli and contain
40.3% in the gills, 31.6% in the intestine and 28.1% in the muscle tissues of Clarias gariepinus.
The concentration of mercury in all the sediments samples ranges from 0.000 – 0.001mg/kg in all the periods of
investigation. The mean concentration of Hg in the sediment from river Guma is presented in figure 5. Similar to
the results of water analysis, the concentrations of mercury in most sediment samples were below the detection
limit. The result shows that mercury level is at pick of maximum permissible level in the soil and any further
addition as a result of anthropogenic activities could leads to contamination.
204

5. Journal of Environment and Earth Science
ISSN 2224-3216 (Paper) ISSN 2225-0948 (Online)
Vol. 3, No.12, 2013
www.iiste.org
Concentration (mg/kg)
0.0012
Hg
0.001
0.0008
0.0006
0.0004
0.0002
0
SAF
SBF
SCF
WHO
Figure 5: Mean concentration of Mercury in sediment from river Guma,
Norville (2005) reported the spatial distribution of heavy metals in sediments from the Gulf of Paria, Trinidad
and recorded mercury concentrations to vary 0.03 – 0.10ppb. Kannan et al (1998) study the distribution of total
mercury and methyl mercury in water, sediment and fish from South Florida Estuaries. They reported total
mercury concentrations in the sediments to range from 1 – 219ng/g dry weight, while methyl mercury accounted
for, on average of 0.77% of total mercury in sediment. The relationship of total and methyl mercury
concentrations in fish to those of sediments from corresponding locations was fish –species dependent, in
addition to several abiotic factors. Kwaansa-Ansah et al (2012) investigated the effect of pH, sulphate
concentration and total organic carbon on mercury accumulation in sediments in the Volta Lake at Yeji, Ghana.
They reported total mercury concentrations ranged from 32.6 – 700ng/g which is below the International Atomic
Energy Agency recommended value of 810ng/g (Coquery et al., 2000). Sizmur et al (2013) sampled sediments
and polychaete worms from mudflats in the Bay of Fundy to investigate the bioaccumulation of mercury and
methyl mercury in the coastal invertebrate food web. Their results shows that mercury concentrations in the
sediments were low (< 20µg/kg) and worms that were feeding deeper sediments contained the greatest methyl
mercury concentrations (69.6µg/kg).
4.0 Conclusion
The mean concentration of mercury obtained in edible frog (0.027mg/kg) was below the International Atomic
Energy Agency recommendation value (IAEA – 433) of 0.168mg/kg. Also, the mean concentrations of mercury
in the edible frog organs and sediments were found to be statistically significant (p = 0.50). This work shows that
edible frog could be used as an excellent bio-indicator of mercury in the aquatic ecosystem as the concentration
of mercury was too low to be detected in water using routine methods. Nevertheless, the gradual accumulation of
mercury within the ecosystem to concentrations of considerable concern was found in edible frog. Generally, the
order of mercury concentration in a descending level in edible frog organs was liver > intestine > muscles; while
the order in the aquatic ecosystem was edible frog > sediment > water.
Acknowledgement
Our appreciation goes to Professor and Mrs Abaa, Mrs Dowuese Ugbidye and Mr Peter Onuwa for their useful
advice and assistance during sampling. We are also grateful to the technical staff of Golden Years Limited Port
Harcourt for their assistance during the analysis.
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6. Journal of Environment and Earth Science
ISSN 2224-3216 (Paper) ISSN 2225-0948 (Online)
Vol. 3, No.12, 2013
www.iiste.org
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